Internal combustion engines have a cylinder block and a cylinder head which, in order to form the individual cylinder, that is to say combustion chambers, are connected to one another, with bores being provided in the cylinder head and in the cylinder block for connection. During assembly, the cylinder block and the cylinder head are arranged relative to one another by placing their assembly end-sides with respect to one another in such a way that the bores are aligned with one another. A connection is then produced by means of threaded bolts which are inserted into the bores of the cylinder head and of the cylinder block.
In order to hold the pistons or the cylinder tubes, the cylinder block has a corresponding number of cylinder bores. The pistons are guided in an axially movable fashion in the cylinder tubes and, together with the cylinder tubes and the cylinder head, form the combustion chambers of the internal combustion engine. A combustion chamber is consequently delimited and configured in each case by a piston, a cylinder tube and the cylinder head together. A seal is generally arranged between the cylinder block and the cylinder head in order to seal off the combustion chambers.
The cylinder head usually serves to hold the valve drive. In order to control the charge exchange, an internal combustion engine requires control elements and actuating devices for actuating the control elements. During the charge exchange, the discharge of the combustion gases takes place via the outlet openings and the charging of the combustion chamber, that is to say the intake of the fresh gas mixture and of the fresh air via the intake openings. In order to control the charge exchange in four-stroke engines, use is made almost exclusively of lifting valves as control elements which perform an oscillating lifting movement during operation of the internal combustion engine and in this way open and close the inlet and outlet openings. The valve actuating mechanism required for the movement of the valves themselves is referred to as a valve drive.
A valve actuating device comprises a camshaft on which a plurality of cams are arranged. A distinction is fundamentally made between a lower camshaft and an overhead camshaft. Here, the reference point is the parting plane between the cylinder head and cylinder block. If the camshaft is situated above said parting plane, it is an overhead camshaft; the camshaft is otherwise a lower camshaft.
Overhead camshafts are likewise mounted in the cylinder head, with a valve drive with an overhead camshaft having, as a further valve drive component, an oscillating lever, a rocker arm or a tappet.
It is the object of the valve drive to open and to close the inlet and outlet openings of the combustion chamber at the correct times, with fast opening of the largest possible flow cross sections being sought in order to keep the throttling losses in the inflowing or outflowing gas flows low and in order to ensure the best possible filling of the combustion chamber with fresh gas mixture and an effective, that is to say complete discharge of the exhaust gases. According to the prior art, combustion chambers are therefore also often and increasingly equipped with two or more inlet and outlet valves.
According to the prior art, the inlet ducts which lead to the inlet openings and the outlet ducts or exhaust-gas lines which adjoin the outlet openings are at least partially integrated in the cylinder head. Here, the exhaust-gas lines of the outlet openings of an individual cylinder are generally merged—within the cylinder head—to form a partial exhaust-gas line which is associated with the cylinder, with said partial exhaust-gas lines then being merged outside the cylinder by means of a so-called (exhaust) manifold; often to form a single combined exhaust-gas line.
Downstream of the manifold, the exhaust gases are then if appropriate supplied to the turbine of an exhaust-gas turbocharger and/or to one or more exhaust-gas treatment systems.
Here, it is on the one hand sought to arrange the exhaust-gas turbochargers as close to the outlet of the internal combustion engine as possible in order to be able to optimally utilize the exhaust-gas enthalpy of the hot exhaust gases and in order to ensure a fast response behavior of the turbocharger. On the other hand, the path of the hot exhaust gases to the different exhaust-gas treatment systems should be as short as possible in order that the exhaust gases are given little time to cool down, and the exhaust-gas treatment systems reach their operating temperature commonly known as the light-off temperature as quickly as possible, in particular after a cold start of the internal combustion engine.
In this context, it is therefore fundamentally sought to minimize the thermal inertia of the part of the exhaust-gas line between the outlet opening at the cylinder and the exhaust-gas treatment system, or between the outlet opening at the cylinder and the exhaust-gas turbocharger. This objective may be contributed to by reducing the mass and the length of said exhaust-gas line.
In order to achieve the above-specified aims, according to one solution according to the prior art, the exhaust-gas manifold is integrated in the cylinder head. A cylinder head of said type, in which each outlet opening is adjoined by an exhaust-gas line and the exhaust-gas lines of the cylinders merge within the cylinder head to form an combined exhaust-gas line, is also the subject matter of the present invention.
A cylinder head of said design is however more highly thermally loaded than a conventional cylinder head which is equipped with an external manifold, and therefore has increased cooling requirements.
The heat which is released during the combustion of the fuel is partially dissipated to the cylinder head and the cylinder block via the walls which delimit the combustion chamber, and partially to the adjoining components and the environment via the exhaust-gas flow. In order to keep the thermal loading of the cylinder head within limits, a part of the heat flow which is conducted into the cylinder head must be extracted from the cylinder head again. The heat quantity which is dissipated from the surface of the internal combustion engine to the environment by means of radiation and thermal conduction is not sufficient for efficient cooling, for which reason cooling of the cylinder head is generally provided by means of forced convection.
It is fundamentally possible for the cooling to be configured in the manner of air cooling or liquid cooling. In the case of air cooling, the internal combustion engine is provided with a fan, with the heat dissipation taking place by means of air flows which are guided over the surface of the cylinder head.
In contrast, liquid cooling requires the internal combustion engine or the cylinder head to be equipped with a coolant jacket, that is to say the arrangement of coolant ducts which conduct the coolant through the cylinder head, which entails a complex structure of the cylinder head construction. Here, the mechanically and thermally highly loaded cylinder head is on the one hand weakened in terms of its strength as a result of the formation of the coolant ducts. On the other hand, the heat need not, as in the case of air cooling, be first conducted to the cylinder head surface in order to be dissipated. The heat is already discharged to the coolant, generally water provided with additives, in the interior of the cylinder head. Here, the coolant is fed by means of a pump which is arranged in the cooling circuit, so that said coolant circulates in the coolant jacket. The heat which is dissipated to the coolant is in this way discharged from the interior of the cylinder head and extracted from the coolant again in a heat exchanger.
On account of the significantly higher heat capacity of liquids in relation to air, it is possible for significantly higher heat quantities to be dissipated with liquid cooling than is possible with air cooling.
For said reasons, according to the prior art, in the case of a cylinder head of the present type, a coolant jacket is integrated in the cylinder head, with the coolant jacket comprising a lower coolant jacket, which is arranged between the exhaust-gas lines and the assembly end-side of the cylinder head, and an upper coolant jacket, which is arranged on that side of the exhaust-gas lines which is situated opposite the lower coolant jacket.
A cylinder head is disclosed in EP 1 722 090 A2, with said European patent application being based on the object of providing as compact a cylinder head as possible, and not a cylinder head with the most efficient cooling action possible.
The cooling of the cylinder head described in EP 1 722 090 A2 has thus proven in practice to be insufficient, with thermal overloading to be expected in particular in the region in which the exhaust-gas lines merge to form the combined exhaust-gas line, which can manifest itself for example in the form of material melting.
In order to prevent this, in an internal combustion engine which is equipped with a cylinder head of said type, an enrichment (λ<1) is always carried out when high exhaust-gas temperatures are expected. Here, more fuel is injected than can actually be burned with the provided air quantity, with the additional fuel likewise being heated and evaporated, so that the temperature of the combustion gases is lowered. This approach is however to be considered disadvantageous from an energetic aspect, in particular with regard to the fuel consumption of the internal combustion engine and with regard to the pollutant emissions. In particular, the enrichment does not always allow the internal combustion engine to be operated in the manner which would be required for a provided exhaust-gas treatment system.
If one also takes into consideration that development towards small, highly-supercharged engines has taken place and continues to take place, it is clear that efficient liquid cooling is of ever greater relevance in practice because the thermal loading in highly-supercharged engines is considerably greater than in convention internal combustion engines.